On a November morning, Olympic rower and financial advisor Harold Backer left for a bike ride and never returned. His disappearance remained a mystery – until letters began arriving at the homes of his investors.

Nearly 20 years ago in a remote California town, a 16-year-old named Karen Mitchell disappeared. The case went cold, but last month local law enforcement started looking at it again after the arrest of a former resident: Robert Durst.

An investigation into the disappearance of a 24-year-old British cruise ship activity director from the Disney Wonder opens the strange and insular world of cruise employees, who vanish mysteriously at alarming rates.